1. Field of the Invention
This invention relates to a viewfinder optical system and more particularly to an optical system for a viewfinder arranged to make an inverted real viewfinder image (object image) formed by an objective lens adequately viewable as a non-inverted erecting image by using an image inverting means, which is appositely set in such a way as to permit reduction in size of the whole optical system.
2. Description of the Related Art
Real-image type viewfinder optical systems have been variously developed to permit a real viewfinder image formed on a primary image forming plane within a viewfinder system of a photographing camera, video camera or the like, to be viewable through an eyepiece. The real-image type viewfinder optical systems permit reduction in size of the whole optical system more easily than virtual-image type viewfinder optical systems. The real-image type viewfinder optical systems, therefore, have been often employed in cameras having zoom lenses.
A real-image type viewfinder optical system disclosed, for example, in Japanese Laid-Open Utility Model Application No. SHO 58-62337 is arranged to convert, by using an image inverting means such as a Porro prism, an object image (viewfinder image) formed by an objective lens on a primary image forming plane into a non-inverted erecting viewfinder image and to make the non-inverted erecting viewfinder image viewable through an eyepiece. Further, Japanese Laid-Open Utility Model Application No. HEI 1-160427 discloses another real-image type viewfinder optical system arranged to invert a viewfinder image by means of a roof-shaped reflection member and two mirrors which are disposed within an optical path.
Furthermore, to meet a demand for reduction in size and thickness of a camera as a whole, Japanese Laid-Open Patent Application No. HEI 6-167739 discloses a viewfinder optical system arranged to shorten its whole lens length by bending at a reflection surface an optical path leading to a primary image forming plane where an object image is to be formed by an objective lens and by forming the primary image forming plane within an image inverting means.
In the conventional real-image type viewfinder optical system of the kind using a Porro prism or the like as an image inverting means for obtaining a non-inverted erecting image, the optical action or an objective lens which forms an object image on a primary image forming plane and that of the image inverting means are clearly divided from each other. The image inverting means, for example, is arranged to exert no refracting power when the object image is formed on the primary image forming plane. Therefore, in a case where a viewfinder optical system has a part of the image inverting means (such as a prism member) between the objective lens and the primary image forming plane, the objective lens is arranged to be of a retro-focus type to make its back focal distance longer. This arrangement necessitates to increase the amount of retro-focusing according to the length of the back focal distance. This requirement has caused the size of the objective lens to be apt to become larger.
In a case where the viewfinder optical system is arranged to have a part of the image inverting means between the objective lens and the primary image forming plane and to have a total reflection surface arranged on the object side of the primary image forming plane to bend an optical path, in order to have a whole light flux efficiently total-reflected by the total reflection surface, the objective lens must be formed by an exit-side telecentric system. That requirement also has caused the size of the objective lens to become larger.